Polyoxometalate-Based Metal-Organic Framework Fractal Crystals

• Published in: Matter Vol. 2; no. 1; pp. 250 - 260
• Main Authors: Zhang, Zhong, Liu, Yi-Wei, Tian, Hong-Rui, Li, Xiao-Hui, Liu, Shu-Mei, Lu, Ying, Sun, Zhi-Xia, Liu, Tianbo, Liu, Shu-Xia
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 08.01.2020
• Subjects: catalysis; fractal growth; hierarchical structure; MAP4: demonstrate; polyoxometalate-based metal-organic framework; catalysis; MAP4: demonstrate; polyoxometalate-based metal-organic framework; hierarchical structure; fractal growth
• ISSN: 2590-2385; 2590-2385
• DOI: 10.1016/j.matt.2019.09.021

Assembling complex hierarchical structures facilely to improve the accessibility of catalytic sites inside bulk catalysts has long been desired but remains challenging. Herein, we fabricate the hierarchical polyoxometalate-based metal-organic framework (POM@MOF) cuboctahedrons by solvent-assisted assembly, and propose a fractal growth mechanism. The great disparity in polarity of the dual solvents used endows them a different strength interaction with the POM. As a result, the polar enriched areas induced by the hydrophilic POM cause diffusion-limited aggregation along with the fractal growth. The obtained POM@MOF fractal crystallites possess an ordered layered surface and mesopores developed from the fractal growth and microscopic separation of the system. The cuboctahedron morphology is directed by the anisotropy of the Keggin units. This is of great significance in constructing hierarchical structures via fractal growth caused by diffusion limit by choosing some solvents with proper polarity disparity, which can be extended to other systems. [Display omitted] •Fractal growth mechanism is proposed in the field of POM@MOFs•POM@MOF fractal crystal with hierarchical pore and surface structures is fabricated•POM induces microscopic separation in the dual solvents with polarity disparity•Cuboctahedron morphology of POM@MOF fractal crystals is directed by Keggin units Improving the accessibility between substrates and the catalytic centers to further enhance the performance of existing catalysts has long been desired and carries great academic and practical significance. Hierarchical POM@MOF crystallites possess more surface catalytic sites and/or mesoporous channels compared with their conventional microporous counterparts, which can greatly enhance the catalysis efficiency. In this report, the hierarchical POM@MOF cuboctahedrons are fabricated and a fractal growth mechanism is proposed. The microscopic separation in the dual solvents with polarity disparity induced by POM causes fractal growth, leading to the obtained fractal crystals possessing ordered layered surface and mesopores, which increases the exposure of active sites and mass transportation along with catalytic performance. The fractal growth and its initiation conditions may be extended to other catalyst synthesis systems. Polyoxometalate-based metal-organic frameworks (POM@MOFs) integrating the merits of POM and MOF are widely used as catalysts. However, the microporous MOF supports often cause accessibility and mass transportation limitations. Assembling hierarchical POM@MOF facilely to improve the accessibility of catalytic POM centers inside remains challenging. Herein, we fabricate the hierarchical POM@MOF cuboctahedrons with ordered layered surface and mesopores by solvent-assisted assembly, and propose a fractal growth mechanism, which may be extended to other systems.